The invention relates to a drive train for a compressor and a hydraulic pump of a motor vehicle which has a hydraulic power steering system and an electric superimposed steering system for providing steering assistance, wherein the compressor and the hydraulic pump are driven by using a common output shaft of a motor vehicle drive motor.
The invention also relates to a method for operating a compressor and a hydraulic pump of a motor vehicle which has a hydraulic power steering system and an electric superimposed steering system for providing steering assistance, wherein the compressor and the hydraulic pump are driven by using a common output shaft of a motor vehicle drive engine.
In modern motor vehicles, in particular in the field of commercial or utility vehicles, systems which are operated with compressed air are used in many cases. For example, the brakes, the pneumatic suspension, the transmission etc. use compressed air as a working medium. In the design and manufacture of such systems it is necessary to keep the power demand for providing compressed air as low as possible while ensuring continuous operational reliability of the systems. Nowadays it is customary to allow the compressor used for providing compressed air to continue to be driven by the engine of the motor vehicle in phases in which it is not feeding any compressed air. The air which is fed at such a time is however not utilized to provide compressed air but rather fed into the surrounding air. This typically causes a power loss of approximately 1 kW.
A further system which is also usually driven by a motor vehicle engine is the hydraulic power steering system. This reduces the steering forces which have to be applied by the driver in the stationary state when maneuvering the vehicle or at low travel speeds of the vehicle. The power demand of such a steering system, also referred to as a power steering system, is approximately 1 kW in the idling state and up to 8 kW in the case of a high demand, for example during maneuvering. In most systems of this type, the hydraulic pump of the steering assistance system is driven by using a through-drive at the compressor. In order to provide the driver with further functionalities in addition to the pure steering assistance, what is referred to as a superimposed steering system has been developed. In this system an additional torque is superimposed on the driver's manual torque, for example by an electric motor and a planetary gear mechanism at the steering column. With this superimposed steering system it is possible to carry out steering corrections in order to stabilize the vehicle as a function of the instantaneous driving state of the vehicle, for example using signals of an electronic stability program (ESP). It is also possible to implement functions such as a variable transmission ratio as a function of the speed of the vehicle or the steering speed as well as a variable power assistance which imposes the superimposed torque as a function of the torque to be applied by the driver.
The exemplary embodiments of the invention provide a drive train for generic applications, so that the compressor and the hydraulic pump can operate in a way which is efficient in terms of energy use.
The drive train according to exemplary embodiments of the invention provides the utilization of elements which reduce the power which is supplied to the compressor and the hydraulic pump via the output shaft, for example of the drive motor, if the demands which are being made of the compressor and of the steering assistance system permit it. This makes it possible for the power to be supplied to the compressor and hydraulic pump in a way which is largely suitable for the demand, thus avoiding unnecessary consumption of energy. In this context it is envisioned, for example depending on the demand profile of the compressor and hydraulic pump, for the power which is to be transmitted to be controllable in a plurality of stages, for example by a transmission or the like. It is also envisioned, if appropriate, in this context for a reduction of the power supplied by the drive shaft to be compensated by an increased power output by the superimposed steering system.
In one particularly preferred embodiment there is provision that the exemplary elements of the system reduce the power which is supplied to the compressor and to the hydraulic pump via the output shaft to zero, if it is not necessary for the compressor to operate and the electric superimposed steering system can provide the necessary steering assistance alone. In particular at a sufficiently high traveling speed there is generally no need for steering assistance, or only a relatively low level of steering assistance is required. This can be applied solely by the exemplary superimposed steering system. If in addition the compressor does not have to provide compressed air, the power flux to the exemplary compressor and hydraulic pump via the output shaft can be interrupted.
In an exemplary embodiment, there is preferably also a provision that the elements of the system include a clutch through which the power flux to the compressor and to the hydraulic pump can be interrupted. Such a deactivation clutch constitutes a cost effective and robust embodiment for disconnecting the compressor and hydraulic pump from the drive engine during suitable operating states. The deactivation clutch can be operated, for example, in idling feed phases in which the compressor would otherwise discharge its output into the surrounding air.
It can also be advantageous that the elements of the exemplary system include an electric control device which determines, on the basis of parameter values fed to it, how far the power which is supplied to the compressor and to the hydraulic pump via the output shaft can be reduced. In this exemplary embodiment, the power flux can be controlled by using specific measurement variables and/or actuation variables which characterize the instantaneous and/or anticipated power demand of the compressor and hydraulic pump. It is also envisioned to take into account further operating conditions of the vehicle such as, for example, the instantaneous driving behavior of the vehicle and parameters for the hydraulics, pneumatics or electrics. In this context the term control also includes control processes.
In particular it may be advantageous for a hydraulic accumulator to be provided which is kept filled in order to be able to make available sufficient steering assistance when demands are suddenly made of the hydraulic power steering system. For example, if the hydraulic pump receives a reduced level of power, or no power, from the drive shaft, it may be necessary to interrupt the power steering assistance by the hydraulics in order to keep the power steering system operationally available for an emergency. In this context it may be advantageous to provide a particularly large hydraulic accumulator in order to cover the time period up to the resumption of operation by the hydraulic pump.
An exemplary method according to the invention includes a system in which the power which is output to the compressor and the hydraulic pump via the output shaft is reduced if the demands which are being made of the compressor and the steering assistance system permit it. In this exemplary embodiment, the advantages of the invention are also implemented within the scope of a method.
The exemplary embodiments of the invention take advantage of the fact that if a vehicle is traveling at a relatively high speed, for example on a freeway, only a small degree of power steering assistance is necessary. This can be provided, for example, solely by the superimposed steering system. It is thus possible to implement a deactivation clutch for the compressor, and thus also for the steering assistance pump. The deactivation clutch may be controlled in an intelligent fashion. It may be activated, for example, if the following conditions are met:
The compressor does not have to feed, i.e. there is no need for compressed fluids.
The demand made of the power steering assistance system by the steering is low, which is determined for example by the pressure in the hydraulic system and by the speed of the vehicle.
The power steering assistance is deactivated by the hydraulics by using a suitable deactivation device in order to keep the hydraulic accumulator filled for an emergency.
In addition, embodiments of the invention may utilize an enlarged hydraulic accumulator in order to be able to reliably cover the time period for switching on the clutch.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.